This invention relates to digital photographic electronics, and more particularly to an automatic tester for camera shutter speeds.
There has been increasing interest in the field of camera test instruments to develop more efficient, easier to use test instruments. The advent of digital electronics and the now readily available micrologic packages with complex processing capabilities has spurred the development of more automated and complex, yet easier to use test instruments. There has been a trend to develop such instruments where possible with digital display elements which are easily read and unambiguous in their meaning.
The most recent examples of the prior art as shown by U.S. Pat. No. 3,603,133 and U.S. Pat. No. 3,760 issued to Westhaver, and U.S. Pat. No. 3,913,376 issued to Scott, have brought the development of shutter testers into the realm of digital electronic processing and are implemented with fixed frequency time bases gated by a light pulse exceeding a threshold point. Some provision is made in one of the Westhaver patents to manually select several time bases according to the anticipated shutter speed range. The selection of a timing period between the half-open to half-closed shutter operation has provided a workable means to interface the analog light sensing circuitry with the digital counting circuitry.
However, the state of the prior art suffers from two shortcomings which are remedied by the present invention. The display, although digital, presents the shutter speed in milliseconds. Any given reading must be interpreted by hand calculation or look-up table to determine the percentage of error present in the shutter for the set speed. Secondly, the use of threshold timing points at the half-open and half-closed shutter positions, although generally producing accurate effective speed readings for focal plane shutter cameras, may produce erroneous results for cameras with between the lens shutters or with nonlinear opening or closing programmed shutters.
The limitations of the one-half open to one-half closed timing method are discussed in the Nov-Dec. 1974 issue (Vol. 5, No. 6) of the SPT Journal, published by the Society of Photo Technologists. A definition of effective exposure time is also presented which requires a technique of integrating the illuminance curve versus time of the light pulse generated by the camera shutter.
The present invention initiates timing processing when the light level striking the photodetector in the focal plane of the camera exceeds one percent of the maximum shutter open light intensity, which has been adjusted during a simple setup operation. Processing continues until the light intensity falls below one percent level again. Timing between these points captures 99.99 percent of the light pulse illuminance curve for processing of the effective shutter speed. The interface between the analog light sensing circuitry and the digital processing circuitry is provided by means of a linear amplifier and voltage controlled oscillator (VCO) combination. By counting the pulses output by the VCO as the shutter operates, the integral of the light intensity versus time illuminance curve of the light pulse is developed. During pretest setup the maximum VCO frequency has been adjusted at full shutter open operation, therefore the timing by this method produce true, effective shutter speed for any camera shutter configuration.
The present invention further processes the VCO output pulse train by means of controllable frequency dividers, presettable shift registers and presettable counters to present the American National Standard Institute (ANSI) standard shutter speed in digital format which most accurately represent the effective shutter speed of the camera shutter tested. An error ratio is also displayed by digital means which directly indicates the performance of the shutter relative to the standard speed displayed.